Italian Pat. No. 1,114,215 describes liquid peroxidic compositions, particularly useful for the cross-linking of polymers (for instance polyethylene or ethylene-propylene elastomers) containing dicumyl peroxide (indicated hereinbelow, for sake of brevity, DCP), a peroxide the use of which (alone) is limited in that, being DCP a solid at room temperature, it is necessary that DCP itself, with the purpose of a more uniform metering (in a continuous operation) be kept in the molten state; such operation often involves a loss of the peroxidic activity and the introduction of impurities. Therefore, the above mentioned Italian Patent suggests the use of balanced peroxy-mixtures, containing an amount of DCP from 5 to 75% together with peroxides which give rise to the formation of liquid mixtures and are comprised in the following general formula (I) ##STR3## wherein R, in meta or para position, is an alkyl group containing from 1 to 3 C atoms. In particular, peroxides where R represents a methyl group are recommended, whereas according to U.S. Pat. No. 4,202,790, it is more advantageous the use of a peroxide where R is an isopropyl group, namely the use of isopropylcumyl-cumyl peroxide, provided the percentage of DCP in the peroxidic mixture be equal to or higher than 25% and preferably 40% by weight. Said U.S. patent advises to prepare the mentioned peroxides separately (and at temperatures above the room temperature) and thereafter to mix the thus obtained peroxides with DCP, according to the desired ratios.
The Applicant has now found that some particular peroxidic mixtures, different from the mixtures described by said U.S. Patent, give rise to better results, in comparison with the preceding mixtures, and can be prepared and treated in a much simpler and more practical way.